1. Field of the Invention
The present invention relates to a lithium ion secondary battery that uses, as a positive electrode active material, a lithium transition metal oxide (also referred to as “lithium-excess transition metal oxide” hereinafter) in which lithium occupies the transition metal site.
2. Description of the Related Art
Due to the increasing performance and functionality of portable appliances, it has been required that secondary batteries used as power sources of such portable appliances have higher capacities. Lithium ion secondary batteries are one type of secondary battery that is expected to meet this requirement. A typical lithium ion secondary battery uses lithium cobaltate as the positive electrode active material and graphite as the negative electrode active material.
Lithium ion secondary batteries of ever higher capacities are expected to be achieved to further pursue advancement in performance and functionality of portable appliances.
Lithium-excess transition metal oxides that contain lithium occupying the transition metal site are drawing attention as a positive electrode active material that can increase the capacity density per weight.
A lithium-excess transition metal oxide represented by Li(LiaMnxNiyCoz)O2 (0<a≦0.34, 0.35≦x<1, 0<y≦0.30, 0<z≦0.30, and 0.95≦a+x+y+z≦1.05) is a solid solution of Li[Li1/3Mn2/3]O2 and Li[NiCoMn]O2. As described in R. Armstrong et al., J. Am. Chem. Soc., 128, 8694-8698 (2006) (Non-patent Document 1), Li[Li1/3Mn2/3]O2 alone can negligibly contribute to charging and discharging but when Li[Li1/3Mn2/3]O2 forms a solid solution with Li[NiCoMn]O2, for example, a relatively large irreversible capacity can be obtained and oxygen desorbs from the active material during initial charging.
Japanese Published Unexamined Patent Application No. 2008-207997 (Patent Document 1) describes that the capacity density of Li[Li1/3Mn2/3]O2 alone can be increased by doping Li[Li1/3Mn2/3]O2 with niobium (Nb).
Japanese Published Unexamined Patent Application No. 2003-68298 (Patent Document 2) describes that the cycle durability is improved by using a lithium transition metal oxide for a lithium secondary battery positive electrode active material represented by LiaNixCoyMnzMpO2 (where 1.00≦a≦1.20, 0.20≦x<0.50, 0.20<y≦0.45, 0.20≦z≦0.50, 0.0005≦p≦0.05, x+y+z+p=1, and M represents a metal element selected from group 4 (4a) elements and group 5 (5b) elements in the periodic table).
Japanese Published Unexamined Patent Application Nos. 2004-161526, 2008-147068, 2008-270086, 2009-110942, 2009-110943, and 2009-110949 (Patent Documents 3 to 8) describe a lithium transition metal oxide represented by LixNi(1-y-z-a)CoyMnzMaO2 (M represents at least one element selected from the group consisting of Fe, V, Cr, Ti, Mg, Al, Ca, Nb, and Zr, and x, y, and z respectively satisfy 1.0≦x≦1.10, 0.4≦y+z≦0.7, and 0.2≦z≦0.5, 0≦a≦0.02), and that incorporation of the metal element M stabilizes the crystal structure.
Japanese Published Unexamined Patent Application No. 2008-153214 (Patent Document 9) describes that when a lithium-excess transition metal oxide represented by yLi[Li1/3Me2/3]O2·(1−y)LiMe′O2 is doped with molybdenum (Mo), titanium (Ti), or the like, the material can have various oxidation numbers and the energy density per volume of an electrode can be increased.
However, none of the above cited documents disclose a method for improving the cycle characteristics, at a high charging voltage, of a lithium ion secondary battery that uses a lithium-excess transition metal oxide as a positive electrode active material from which oxygen desorbs during initial charging.